基于对网络安全性的需求，网络操作系统一般采用四级安全保密机制，即注册安全、用户信任者权限、对答信任者权限屏蔽与(64)。

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In　the　following　essay,　each　blank　has　four　choices.　Choose　the　best　answer　and　write　down　on　the　answer　sheet.In　low-speed　network,　it　is　usually　adequate　to　wait　for　congestion　to　occur　and　then　react　to　it　by　telling　the　source　of　packets　to　slow　down.　In　high-speed　networks,　this　approach　often　works　poorly,　because　in　the(71)between　sending　the　notification　and　notification　arriving　at　the　source,　thousands　of　additional(72)may　arrive.　In　ATM　network,　a　major　tool　for　preventing(73)is(74)control.　When　a　host　wants　a　new　virtual(75),　it　must　describe　the　traffic　to　be　offered　and　the　service　expected.

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For each blank, choose the best answer from the four choices and write down on the answer sheet.

 (66) is a one-way function that takes an arbitrarily long piece of plaintext and from it computes a fixed-length bit string.

 (67) is a protocol that most USENET machines now use it to allow news articles to propagate from one machine to another over a reliable connection.

 (68) is an approach preferred by cable TV providers. In this solution the fiber terminates at an optoelectrical converter and the final segment is a shared coaxial cable.

 (69) is a device used to connect two separate network that use different communication protocol.

 (70) is a digital-to-digital polar encoding method in which the signal level is always either positive or negative.